1. Field of the Invention
The present invention relates to an image forming apparatus that employs an electrophotographic method to form an image.
2. Description of the Related Art
The primary modes of a transfer system pertaining to a conventional image forming apparatus of a tandem system are a direct transfer system and an intermediate transfer system, the latter of which employs an intermediate transfer member to perform a secondary transfer. The direct transfer system comprises a feeding belt, which carries and conveys a sheet of printing paper. Yellow (Y), magenta (M), cyan (C), and black (Bk) process cartridges (“cartridges”), are placed in tandem, aligned along a feeding direction of the feeding belt. An optical unit is installed thereupon, corresponding to each cartridge. A transfer roller is also positioned that sandwiches the feeding belt, corresponding to a photosensitive drum, i.e., an image carrier, of each respective cartridge. Given such a configuration, yellow, magenta, cyan, and black toner images, which are obtained via an established electrophotographic method are overlaid upon, and transferred to, the sheet of printing paper that is supplied from a printing paper cartridge to the feeding belt. The toner image that is transferred to the sheet of printing paper is fixed by a fixing unit, and discharged from the apparatus via an output sensor and a conveyor path.
When forming a toner image on the reverse side of the sheet of printing paper as well as the obverse, the sheet of printing paper that is discharged from the fixing unit is conveyed once more to the feeding belt via another path, and the image is formed on the reverse of the sheet of printing paper via a sequence of steps similar to the foregoing. The feeding belt is driven by a conveyor drive roller thereof. The drive motor of the feeding belt is driven to rotate in a set speed of rotations in order to obtain a high quality image.
The intermediate transfer system, on the other hand, possesses an intermediate transfer belt, whereupon a primary transfer of images that are formed in the photosensitive drums is performed, and the image that has been primarily transferred on the intermediate transfer belt is further transferred to form on the sheet of printing paper via a secondary transfer. The drive motor of the intermediate transfer belt is driven to rotate in a set speed of rotations in order to obtain a high quality image.
When both of the transfer systems are concerned, such factors as control of the temperature of a heater within the fixing unit, or heat emitted by the various drive motors, causes the temperature to rise within the image forming apparatus, with the feeding belt or the intermediate transfer belt experiencing either thermal expansion or contraction, and the speed of conveyance speeding up and slowing down, resulting in a lack of uniformity thereof. Consequently, a misalignment in color from a specific position of the sheet of printing paper may occur when each respective color toner image is transferred, resulting in significant degradation in quality of the image thus formed. Control of the conveyance of the feeding belt or the intermediate transfer belt involves controlling the rotation of the drive motor so as to maintain a constant fixed speed, and thus, a virtual radius of the feeding belt or the intermediate transfer belt being altered by thermal expansion may lead to a lack of uniformity in surface speed thereof, and a resulting misalignment in color.
U.S. Pat. No. 6,655,774 recites a method of solving such a problem by scanning an image on the sheet of printing paper, the feeding belt, or the intermediate transfer belt, deriving the speed in motion thereof, and flexibly controlling the rotational speed of each respective drive motor.
The configuration of U.S. Pat. No. 6,655,774 is limited, however, to a belt surface region, i.e., a number of pixels in a detected image, which can be detected in a single sample. The following is a description thereof.
FIG. 1 describes the related art.
FIG. 1 depicts an area sensor 100, which possesses a sensor element with dimensions of m pixels in the moving direction of the intermediate transfer belt (or feeding belt or sheet of printing paper, hereinafter collectively “belt”) 101, and n pixels in the perpendicular orientation thereto. In the present circumstance, the belt 101 moves in the direction denoted by reference numeral 102. A of FIG. 1 depicts a top view, and B of FIG. 1 depicts a side elevation view.
A target pattern is determined from an image 103 that is sampled at a time t, with a barycenter of the pattern treated as a target 104. The trajectory of the target 104 is sampled at a predetermined sampling rate, and the speed at the surface of the belt 101 calculated from the distance thus sampled. The moving speed at which the belt 101 moves is subject to fluctuation depending on the type of sheet of printing paper. Using a thick sheet of printing paper or cardboard, for example, reduces the process speed, i.e., the speed of image forming, compared with a sheet of plain printing paper, in order to increase the fixing characteristic thereof. A problem that arises as a result is the relation between an area of detection and the processing time. The following is a description of what happens with (a) a slow moving speed, i.e., thick paper/cardboard mode; (b) a fast moving speed, i.e., plain printing paper mode; and (c) when the area of detection is expanded due to the fast moving speed.
FIGS. 2 through 4 describe detecting a moving speed of a belt in a conventional manner.
Reference numeral 111 in FIG. 2, i.e., slow moving speed, denotes an image that is detected by the sensor 100 at a time t1. Reference numeral 112 denotes an image that is detected by the sensor 100 at a time t2, where t1<t2. The overall detection area of the sensor 100 detects the images 111 and 112 at different timings. Reference numeral 113 denotes a target. Va denotes the moving speed of the belt 101 at the time depicted. The relation “e2−e1” is the processing time period for the image 111 that is scanned by the sensor 100 (“e1 indicated a start timing of the processing and “e2” indicates an end timing of the processing). The time period is shorter than the interval for sampling from the time t1 to the time t2, “t2−t1”. The example depicted in FIG. 2 contains the distance within the sampling interval, i.e., 4=6−2, within the area of the sensor 100. The image 111 is detected at the time t1, and the coordinates Y1=2 of the target 113 of the image 111 thus detected are processed at the time e1, after which the image 112 is detected at the time t2. A target 113′ may be identified within the image 112, and the coordinates Y2=6 thereof detected. It is thus possible to derive the distance of the belt (Y2−Y1=4) between the time t1 and the time t2.
FIG. 3 represents an example of detection when the moving speed Vb of the belt 101 is rapid, and Va<Vb. Reference numerals 121 and 122 denote images that the overall detection area of the sensor 100 detects at different timings. The image 121 is detected at the time t1, and the coordinates Y1=2 of the target 113 of the image 121 thus detected are processed at the time e1, after which the image 122 is detected at the time t2. In such a circumstance, however, the fast speed of the belt 101 means that the target 113 protrudes into reference code U, which is outside the sensor area 100, and thus, the target 113 cannot be identified within the image 122. Consequently, the distance of the belt between the time t1 and the time t2, i.e., Y2=m+4, cannot be detected and thus, the distance (Y2−Y1) cannot be derived. If the moving speed of the belt is too fast, the target 113 will have already passed the sensor area by the time the next sampling timing arises, precluding detection of the speed of the belt 101.
FIG. 4 depicts an example of expanding an area 114 that is detected by the sensor 100 at the speed Vb that is depicted in FIG. 3. Reference numerals 131, 132, and 133 denote images that the overall detection area of the sensor 100 detects at different timings.
FIG. 3 depicts the detection of the image 121 at the time t1, and processing the coordinates Y1=2 of the target 113 in the image 121 thus detected at the time e2. In FIG. 4, the sensor area wherein an image 131 is detected at the time t1 is expanded, thus increasing the number of pixels handled thereby, and reducing the processing speed below the processing speed depicted in FIG. 3. Consequently, the processing up to the identification of the target 113 in the image 131 would not finished at the time t2 for the next sample, i.e., e2>t2. Hence, a target 113′ will be within the sensor area 114 at the time t2, image processing will not be finished in time, owing to the increased number of pixels, and thus, the target 113′ cannot be detected in the image 132. The sampling interval is thus presumed to be extended to a time t3 in order to allow for a sufficient time for processing of the image 131, where e2<t3. In such a circumstance, accuracy of detection of speed declines, and a target 113″ falls into U, the region outside the sensor area 114, at the time t3, preventing identification of the target 113″ within the image 133. As a result, the coordinates Y2=m+10 cannot be detected, and thus, the distance (Y2=Y1) of the belt between the time t1 and the time t3 cannot be derived. Raising the speed of the belt in such a manner thus prevents detecting the distance of the belt, even if the sensor area is expanded.